(a) Field of the Invention
The present invention relates to a microwave separator for separating a microwave signal to those having frequencies included in two frequency bands.
(b) Prior Art
The car telephone employs the two-way communication system which enables transmission and reception to be achieved using the microwave signal at a band of 800 MHz. In the case of this car telephone, communication is carried out using a transmission signal included in a transmission frequency band of 825-851 MHz and a reception signal included in a reception frequency band of 870-896 MHz. A microwave separator for 870-896 MHz. A microwave separator for separating these transmission and reception signals is arranged at an input/output section of the car telephone.
FIG. 4 is a block diagram roughly showing the microwave separator. A transmission signal input terminal 76 is connected to one end of a first band-pass filter 73 (which will be hereinafter referred to as BPF) and which takes the transmission frequency band as its pass band, while an end of a coaxial cable 78 having a wavelength which is a quarter of that of the center frequency in the transmission frequency band is connected to the other end of the BPF. A reception signal output terminal 77 is connected to one end of a second BPF 74 which takes the reception frequency band as its pass band, while an end of a coaxial cable 79 having a wavelength which is a quarter of that of the center frequency in the reception frequency band is connected to the other end of the second BPF 74. The other ends of the coaxial cables 78 and 79 are connected to an antenna terminal 75. The coaxial cables 78 and 79 are used to reduce those undesirable influences which the first and second BPFs 73 and 74 exert on each other.
The BPFs 73 and 74 are needed to have a sharp skirting characteristic because their pass bands are wide and because the transmission and reception frequency bands are close to each other. The car telephone and other such equipment need to be small-sized and light-weighted.
Therefore, filters of the coaxial or comb line type are conventionally used as the first and second BPFs for the microwave separator.
The filter of the coaxial type will be briefly described referring to FIGS. 5 and 6, of which FIG. 5 is a perspective view showing the filter of the coaxial type and FIG. 6 is a circuit diagram showing the filter.
In FIG. 5, the coaxial type filter comprises providing inner conductors 86-90 on inner walls of resonators 80-84 of the coaxial type, respectively, which are cylindrical dielectrics, locating the resonators 80-84 between terminals 97 and 98, and connecting the inner conductors 86-90 in series to the terminals 97 and 98 through capacitors 91-96.
The circuit of this coaxial type filter has an arrangement of the capacity coupling in which the coaxial type resonators 80-84 are connected in series to one another through the capacitors 92-95, as shown in FIG. 6. The skirting characteristic is sharp at low band, but attenuation is small and the skirting characteristic is gentle at high band in this case. Numerals 99-103 represent fringing capacities and numerals 104-108 equivalent coaxial lines.
The filter of the comb line type will be described referring to FIGS. 7 and 8, of which FIG. 7 is a perspective view showing the filter of the comb line type and FIG. 8 a circuit diagram showing the filter in FIG. 7.
In FIG. 7, the comb line type filter comprises providing holes in a rectangular-parallelepiped-shaped dielectric 114 along a center line thereof with an equal interval interposed between them, forming inner conductors 109-113 of plating on inner walls of the holes, providing conductors on those faces of the dielectric 114 except the top face thereof at which the holes are opened, and connecting the inner conductors 109 and 113 located on both sides to terminals 117 and 118 through capacitors 115 and 116.
The circuit of this comb line type filter comprises inductively coupling resonators by equivalent coaxial lines 119-122, and the skirting characteristic is sharp at high band but attenuation is small and the skirting characteristic is gentle at low band in this case. Numerals 123-127 represent fringing capacities and numerals 128-132 equivalent coaxial lines.
In the case of the conventional microwave separator, either of the coaxial type filters or comb line type filters are used as the first and second BPFs 73 and 74.
FIG. 9 shows the characteristic of the microwave separator in which the coaxial type filters are used as the first and second BPFs 73 and 74. As apparent from FIG. 9, the skirting characteristic is not sharp enough at high band relating to a property A of the first BPF 73 in the case of this microwave separator provided with the coaxial type filters and it overlaps the skirting characteristic which is related, at low band, to a property B of the second BPF 74. Return loss of the second BPF 74 is thus worsened, as shown by D in FIG. 9, thereby causing the receiving sensitivity to be made worse.
FIG. 10 shows the characteristic of the microwave separator in which the comb line type filters are used as the first and second BPFs 73 and 74. As apparent from FIG. 10, the skirting characteristic is not sharp enough at low band relating to a property F of the second BPF 74 in the case of this microwave separator provided with the comb line type filters and it overlaps the skirting characteristic which is related, at high band, to a property E of the first BPF 73. Signals at the transmission frequency band are therefore likely to leak into the reception frequency band, so that normal operation cannot be carried out on the reception side to thereby lower the receiving sensitivity. Further, the return loss of the first BPF 73 become worse, as shown by G in FIG. 10, radio wave emitted from the antenna is weakened, and heat value is increased at the final stage on the reception side.
C, D in FIG. 9 and G, H in FIG. 10 represent the return losses of the first and second BPFs 73 and 74 obtained when they are of the coaxial type and of the comb line type, respectively.
The number of stages of the resonators can be increased to make the skirting characteristic sharp, but when so arranged, signal loss is increased accordingly in the pass band, thereby making it difficult to adjust the resonators. This measure is not therefore practical.